Precisely measuring the concentration of a component of a sample gas mixture under flow conditions can be difficult, particularly when the sample is at high temperature and contains high concentrations of carbon dioxide and water vapor. A solution to this problem is to rapidly dilute the sample in air or inert gas prior to analysis.
Commercial measurement equipment typically includes a probe having a critical orifice within the housing of the probe to control the flow rate of sample into a diluent gas. A stream of a sample gas enters the probe and flows through the critical orifice at sonic velocity, that is, the velocity of sound in the gas mixture. The sample gas then flows into the diluent gas, and is diluted by a presumably known dilution ratio. The diluted sample gas is then routed to an analyzer which is in communication with the probe. The analyzer provides a reading concerning the concentration level of the component in the diluted sample gas.
To calibrate the analyzer prior to measurement of the component, a calibration gas having a known concentration of the component is used to verify concentration readings from the analyzer. During the calibration of the analyzer, the calibration gas also flows through the critical orifice of the probe at sonic velocity prior to dilution. Difficulties can be encountered with current measurement methods, particularly when the calibration gas includes a plurality of components (a multi-component calibration gas), wherein different components of the same calibration gas are used to calibrate different analyzers. These difficulties arise because the velocity of sound can vary significantly from one gas to another gas and therefore from one gas mixture to another gas mixture.
Although methods for measuring concentration levels of components of diluted sample gas are well known in the art, known prior art methods are deficient in providing consistent, reliable data for commercial use in some measuring applications. In known methods, the difference in the respective flow rates of the calibration gas and the sample gas through a critical orifice is simply assumed to be negligible, and therefore not accounted for in the analysis. Although the flow rate of a gas mixture is dependent on numerous factors, it is possible to approximately compensate for the difference in the flow rates by a mathematical factor in the calculation. In either instance, less than exact measurements are achieved. To address this problem in prior art methods, it has not been known until the present invention to physically affect the calibration gas so that the calibration gas has the same or similar flow rate through the critical orifice as the sample gas which is to be measured, or some other gas mixture to be used as part of the calibration process.
It is therefore a goal of the present invention to provide a method of controlling the flow rate of a gas mixture through a critical orifice to match the flow rate of a sample gas or some other selected gas or mixture of gases. It is a further goal of the invention to provide a calibration gas for use in calibrating an analyzer, wherein the calibration gas includes an additional gas or gases, the additional gas affecting the flow rate of the calibration gas through a critical orifice, but otherwise not affecting the analysis. It is a further goal to provide a calibration gas having a flow rate that is equal to the flow rate of the sample gas, or other selected gas, through the critical orifice at sonic velocity.